1. Technical Field
This invention relates generally to an exhaust system having a controllably variable flowpath for exhaust gas circulation, and more particularly to the use of such a system to control exhaust gas flow through regenerable exhaust gas treatment devices.
2. Background Art
Worldwide emissions regulations slated for introduction in the near future impose very stringent emissions regulations. The Tier 2 regulations in the United States require that Diesel vehicles have the same ultra-low emissions levels as spark ignited vehicles. Moreover, Tier 3 requirements, which phase in for different engine levels over the next three years call for a 40% reduction in NOx (oxides of nitrogen) from the Tier 2 levels now in existence.
Various combustion modes, directed to addressing both in-cylinder (engine-out) and exhaust gas treatment device requirements, have been proposed. For example, U.S. Pat. No. 5,732,554, issued Mar. 31, 1998 to Shizuo Sasaki, et al. for an EXHAUST GAS PURIFICATION DEVICE FOR AN INTERNAL COMBUSTION ENGINE describes a method by which the normal fuel lean operating mode of an engine is switched to a rich premixed charge compression ignition, more accurately and preferably referred to as premixed controlled compression ignition (PCCI), combustion mode.
U.S. Pat. No. 5,937,639 granted Aug. 17, 1999 to Shizuo Sasaki, et al. for INTERNAL COMBUSTION ENGINE describes an alternative method for lowering the combustion temperature, i.e., low temperature combustion (LTC) to minimize smoke generation during rich, or near rich, combustion. LTC and PCCI combustion are alternative combustion modes which normal Diesel lean combustion can be transitioned to during engine operation.
Perhaps of most concern to the Diesel engine market are the proposed very tight future reductions in terms of oxides of nitrogen (NOx) and particulate matter (PM) emissions. One of the most promising technologies for NOx treatment is NOx adsorbers, also known as “lean NOx traps.” Diesel particulate filters, also known as Diesel particulate traps, and lean NOx traps are the most likely, at least in the foreseeable future, means by which emissions will be reduced. Lean NOx traps and Diesel particulate filters need to be regenerated periodically to restore their efficiencies. The regeneration of lean NOx traps is usually done by providing reductants, such as CO and HC under oxygen-free conditions. A regenerated lean NOx trap not only adsorbs NOx emissions, but also adsorbs sulfur carried in the exhaust gas stream. Sulfur removal (desulfation) must be undertaken at a temperature above 600° C. under oxygen-free conditions, i.e., combustion of a stoichiometric or richer air/fuel ratio. Under typical Diesel lean combustion operation, such very high temperatures cannot normally be obtained except under very high load conditions. Diesel particulate filter regeneration is carried out by oxidizing soot and other particles “trapped” in the Diesel particulate filter at a high temperature and a lean air/fuel ratio.
Frequent lean NOx trap (LNT) regeneration is necessary when the engine-out NOx is high, for example, when operating under high loads, but frequent generation at higher loads can cause the temperature of the LNT to increase rapidly. The rapid temperature increase results from the exothermic reaction associated with the rich combustion products carried in the exhaust that are used to regenerate the LNT. Diesel particulate filters (DPF) also require high temperature to be regenerated. When the LNT is located downstream of the DPF, the exothermic reaction taking place in the DPF during regeneration will result in an increase in the outlet exhaust gas temperature of the DPF. The LNT inlet exhaust gas temperature is therefore also increased. When the temperature of the LNT increases above a critical temperature, as a result of the frequent regeneration of the LNT or the regeneration of the DPF, the absorption efficiency of NOx by the LNT is very low and tailpipe NOx emissions accordingly are high.
The present invention is directed to overcoming the problems set forth above with respect to the critical temperature requirements associated with catalyzed and other exhaust gas aftertreatment device operation and regeneration. It is desirable to have an exhaust system in which LNTs, selective catalytic reduction (SCR) catalysts, and like regenerable exhaust gas treatment devices can be optimally positioned within the exhaust system to provide efficient operation over a wide range of engine operating conditions and regeneration requirements. It is also desirable to have a flexible exhaust system in which the exhaust gas flowpath can be selectively varied to control regeneration temperatures to meet differing operation and regeneration requirements. It is also desirable to have a method by which LNT temperature can be managed for the best efficiency during cold start, regeneration and DPF regeneration. It is also desirable to have an exhaust system and method of temperature control by which unregulated emissions can be reduced by management of the exhaust gas temperature passing through various components of the exhaust system.